This invention relates to an improved head actuator.
A tape drive for digital computer use demands great precision of its tape head actuator or positioning system, but head positioning systems are required to work in an "open loop" environment. And in an "open loop" tape drive environment there is no feed back from the head positioning system, or any other device, to keep the head centered on a tape track during operation. Consequently, the mechanical mechanisms of these head positioning systems must be exceedingly tight and stable. But the design of prior art tape head actuators have not met the task.
In operation, prior art actuators have been plagued by two major problems: backlash and tape head off-set in the azimuth direction (azimuth tilt). In operation these deficiencies produce poor alignment that causes problems such as increased noise, reduced signal strength, and increased error rate.
Regarding azimuth tilt, it is pointed out that during operation magnetic tape streams over a tape head at extremely high velocities. And at these high velocities tape movement exerts dynamic force on the tape head actuator mechanism that changes when tape direction changes. These direction changes cause a tape head to rock back and forth creating head off-set or azimuth tilt.
FIGS. 1 and 2 illustrate one prior art tape head actuator approach. FIGS. 1 and 2 show an actuator 5 that includes a tape head bracket 10 carried on the lead screw output shaft 12 of a stepper motor 14 by a partial nut 16 that engages the shaft 12. The lead screw shaft 12 extends through spaced apart bearing type guide passageways 18 and 20 formed in regions 22 and 24 of the bracket 10. As shown, the partial nut 16 engages the lead screw shaft 12 between the regions 22 and 24 under the influence of a biasing leaf spring 26. As shown, the leaf spring 26 is mounted at its ends on the bracket 10 and is bent over the nut 16 at its mid-region. Hence the leaf spring 26, which has a relatively steep force-deflection curve, urges or pre-loads the nut 16 against the lead screw shaft 12. But this urging of the nut 16 against the shaft 12 by the leaf spring 26 causes reactionary forces on the bearing surfaces of the guide passageways 18 and 20 that induce frictional drag--and consequently wear.
The physical arrangement of the actuator 5 and the use of a leaf spring contribute to backlash and azimuth tilt. As indicated above, leaf springs have a relatively steep force-deflection curve. That is, for small change in deflection a leaf spring produces a large change in force. Hence, as the nut 16 and the bearing surfaces of the guide passageways 18 and 20 wear, there is considerable reduction in biasing, or pre-loading, force of the leaf spring 26 on the nut 16 against the lead screw shaft 12. As a result there is a reduction in the effectiveness of the inter-engagement of the nut 16 and the shaft 12 that worsens backlash. Moreover, the decreased biasing force also reduces the effectiveness of the mechanism under the influence of external forces, which makes the actuator 5 susceptible to azimuth tilt.
Other prior art actuators use an actuator arrangement like that shown in FIGS. 1 and 2, but with variations in how the partial nut 16 is biased against the lead screw output shaft 12 of the stepper motor 14. For example, a cantilever mounted leaf spring arrangement has been used to bias a partial nut. Also, compression springs have been used to bias the partial nut when space permits. But each of these prior art arrangements produces reaction forces at the guide passageways, and wear makes the mechanisms more susceptible to backlash and azimuth tilt.
There remains a need for a tape head actuator that provides tracking accuracy. And in an "open loop" tape drive environment this means a tape head actuator that operates essentially free from backlash and azimuth tilt.